The present invention relates to a flow control system, and in particular to a system for controlling the flow of a mixed phase fluid through a vessel. The term xe2x80x9cmixed phase fluidxe2x80x9d is used to cover fluids made up of for example suspended particulates, liquids emulsions and gas derived from different constituents for example oil and water, or liquid and gas derived from the same constituent.
Oil field production systems generally comprise separator plant in which raw fluid pumped from an oil bearing formation is separated into its constituent parts, that is volatile gases, liquid petroleum products, water and particulates. The nature of the input fluid to the separator plant can vary widely over relatively short periods of time. For example a large proportion of the flow may be made up of water for a first period of time and oil and gas for a second period of time. It is difficult with a separator of fixed configuration to satisfactorily process different flows when flow conditions change in an unpredictable manner.
In a conventional separator, an inlet flow is generally passed through a stack of inclined plates within a relatively large vessel, the inclined plate encouraging the separation of water, oil and gas into separate superimposed layers. Gas can then be extracted from an upper section of the separator vessel and the water and oil can be separated by a simple weir separator plate the height of which is arranged to be above the interface between the water and oil layers. If the input flow is such that the separator plates become largely filled with a foam or emulsion of for example oil and water the separation performance is significantly degraded. Similarly, if a large volume of water is delivered to the separator in a relatively short period of time, it can be difficult to maintain the water/oil interface below the level of the weir separator.
With such problems in mind, the normal approach to separator design has been to provide a relatively large capacity separator which is capable of dealing with a wide range of conditions by in effect accepting wide fluctuations in separator plate efficiency and water/oil interface levels. As a result of this design philosophy, separator plant can make up a significant proportion of the size and weight of oil field equipment. This is a particular problem in the case of offshore oil fields where the size and weight of offshore equipment determines the economic viability of some oil bearing formations.
Attempts have been made to monitor separator performance in particular circumstances so as to be able to match separator design to expected separator operating conditions. The equipment used has generally required the mounting of heavy gamma ray sensors on separator equipment. The use of such equipment for monitoring routine operating conditions is not appropriate.
Extensive work has been conducted to enable flow conditions within for example circular-section pipes to be monitored. For example, U.S. Pat. No. 5,130,661 describes a capacitance sensor system in which an array of capacitor plates is disposed around the outer periphery of a pipe through which a mixture of oil, water and gas is passing. By appropriate manipulation of output signals derived from the sensors, an image can be built up of the flow cross-section. Such sensing systems have been used for example to estimate mass flow rates of different phases but it has not been suggested that the output of such systems can be used to achieve real-time process control.
It is an object of the present invention to provide a fluid flow control system which obviates or mitigates the forementioned problems.
According to the present invention, there is provided a system for controlling the flow of a mixed phase fluid through a vessel, comprising at least one flow control device located in an Inlet to or outlet from the vessel, a plurality of sensors distributed about the vessel to sense characteristics of the flow adjacent the sensors, means for monitoring outputs of the sensors to detect the location of a boundary between phases within the flow, and means for controlling the flow control device to maintain the boundary location within pre-determined limits.
The present invention may be applied to the control of processes such as inclined plate separation and weir plate separation. The supply of fluid to or the extraction of different phases from the vessel can be controlled so as to maintain interfaces between different phases at desired levels. For example, in the case of a weir plate, used to separate water and oil, the rate at which water is removed can be controlled to maintain the water/oil interface above a water outlet and below an upper edge of the weir. Similarly, the supply of fluid to an inclined plate separator can be controlled to prevent the separator becoming ineffective for example as the result of the build-up of emulsions between the plates.
Preferably the sensors are embedded in vessel surfaces adjacent to which the fluid flows, for example in the surfaces of the plates of inclined plate or weir plate separators. The sensors may be capacitance sensors, pressure sensors or conductivity sensors for example. In the case of an inclined plate separator, the sensors may be supported in for example three vertical arrays, the arrays being spaced apart in the directions of fluid flow through the separator.
The sensors may be provided in a multilayer structure comprising a top layer of conductance sensors, a screening layer, and a layer carrying a series of connections, each layer being separated from the other by dielectric material, conducting paths between the sensors and the connections passing through the screening layer. This arrangement is advantageous because it avoids xe2x80x9ccross-talkxe2x80x9d between the sensors and the connections. This makes conductance measurements more accurate, and allows some sensors to be used as sources of electric fields whilst other sensors are simultaneously used as detectors of electric fields.
The screening layer preferably comprises a layer of conducing material. Screening conducting material is preferably located between the connections to prevent cross-talk between the connections. A most preferably way of providing the multi-layer structure is by bonding together a series of printed circuit boards.
The sensors may be supported in a vertical array, for example at vertically spaced positions on a weir separator, or at vertically spaced positions on an elongate support extending vertically through the fluid flow. The sensors may be arranged to have a common axis or lie in a common plane, so that measurements of electrical properties are made between adjacent sensors.
The elongate support may comprise a rod having two faces subtending an angle of less than 180 degrees, the faces being provided with an array of sensors. This is advantageous over known configurations of sensors because it reduces the possibility of solid material becoming trapped between opposing sensors.
Preferably, at least one sensor is capable of acting either as a source of an electric field or as a detector of an electric field. Providing each of the sensors with this capability allows the distribution of detectors and sensors to be optimised for any required measurement.
Preferably, at least one of the sensors is located on a dielectric mounting. Where the dielectric mounting forms a casing in one surface of which a sensor is located, an electric field obtained by applying a voltage to that sensor will retain its general shape in the event that the permittivity of the media surrounding the casing is increased.
Preferably, at least one of the sensors is separated by dielectric material from an electrical connection to another of the sensors. This allows some sensors to be used as detectors and some to be used as sources of electric fields.
The sensors may control for example a device for injecting flow control chemical additives, or a flow choke device. Such a flow choke device may be located upstream from or downstream from a T-junction defining one inlet and two outlets, the flow choke being controlled to control separation processes occurring at the junction Thus an inlet flow to for example an inclined plate separator can be pre-conditioned by diverting at least a proportion of unwanted components away from the inclined separator at a T-junction located upstream of that separator.
The invention also provides a system for monitoring the flow of a mixed phase fluid through a vessel the vessel defining surfaces adjacent to which the fluid flows, wherein sensors are embedded in the said surfaces, each sensor providing an output representative of a characteristic of the flow adjacent the surface within which it is embedded, and the sensors being distributed within the said surfaces such that the location relative to the vessel of a boundary between phases of a flow can be determined from a comparison between the sensor outputs.
The invention also provides a method of calibrating an array of sensors configured to monitor the location of a boundary between layers of fluid, the method comprising obtaining a series of measurements from two of the sensors while the boundary between the layers of fluid is at a series of locations between central areas of the sensors. Once the array of sensors has been calibrated for a given fluid or fluids, the position of the boundary between the layers of the fluid or fluids can be very accurately determined.